Organic electroluminescent (OEL) devices such as organic light emitting diodes (OLEDs) are desirable for use in electronic media because of their thin profile, low weight, capability of obtaining a wide variety of emission colors, and low driving voltage. OLEDs have potential use in applications such as backlighting of graphics, pixelated displays, and large emissive graphics.
There is continuing research and development of electroluminescent materials, electroactive materials, and charge transporting materials suitable for such devices and methods for making the devices. In some instances, materials can be selected or developed which facilitate one or more of these device preparation methods. Pattern-wise thermal transfer of materials from donor sheets to receptor substrates has been proposed as one method for forming OEL devices. Selective thermal transfer of organic light emitters for formation of organic electroluminescent devices has been shown to be particularly useful.
Molecular oxadiazole and triazole derivatives have been used as electron transport/hole blocking materials in OLED devices. One oxadiazole derivative commonly used is 2-(4-biphenyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD). One triazole derivative commonly used is 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)1,2,4-triazole (TAZ). However, OLED devices employing PBD or TAZ often exhibit short operating lifetimes due to recrystallization of or aggregate formation by the PBD or TAZ, leading to phase separation and formation of charge carrier traps that inhibit emission.
In an effort to mitigate these problems in blended polymer systems, several groups have reported bonding an electron transporting structure such as PBD to a flexible polymer chain, resulting in amorphous materials. For example, polymethylmethacrylates bearing oxadiazole side chains have been reported. However, operating lifetimes for devices based on these materials were found to be extremely short due to PBD aggregation (e.g., see Strukelj et al., Science, 267, 1969, (1995)).